Climate change and human societies

The possible interactions between climate change and societal changes were discussed over 50 years ago by von Post (1946 - see above) and this topic continues to be a critical issue in Holocene climate research (Oldfield 2005, this volume). Recent impetus to understanding more fully the interactions between climate and humans (Oldfield 2005, this volume) has come from (i) the demonstration of major changes in the hydrologic balance in different areas, especially at low latitudes, leading to periods of extended drought (see above; Haberle and Chepstow-Lusty 2000; Verschuren and Charman, this volume), and (ii) Ruddiman's (2003, 2005b) Anthropocene hypothesis concerning the possible role of land-cover and land-use changes on atmospheric concentrations of carbon dioxide and methane and hence on Holocene climate (see above).

There are many examples where prolonged drought episodes detected by paleoecologic studies coincide with and may have been one of the dominant factors contributing to major declines or "collapses" in civilizations such as the Maya and Anasazi, and in areas such as the Atacama and Andean Antiplano, the Sahara, eastern Mediterranean, east Africa, South Africa, and China (Oldfield 2005, this volume). Several recent books explore the idea of the impacts of climate change on societies (e.g. Fagan 1999, 2000, 2004; Burroughs, 2005; Linden, 2006). Oldfield (2005, this volume) emphasizes that this type of research is extremely complex. Balanced and critical analyses are needed where climate changes and cultural perspectives are considered as an integrated whole (Diamond 2005). The influence of climate change on human societies is mediated by complex cultural and social processes. There is thus a need to try to understand the nature of these processes, especially during periods of rapid climate change and limited resources. There is currently a strong polarization of ideas in this area. Some interpret past changes in human societies such as the "collapse" of different civilizations to be the direct consequence of climate change, particularly extreme and/or rapid events (e.g. deMenocal 2001). Others see these societal changes as a result of changes in societal organization and of human processes that may have developed independently of climatic influences (e.g. Redman 1999). This strong polarization of ideas has resulted in the complex nature of human-climate interactions in the past being ignored and a failure to improve our understanding of the effects of these interactions on ecosystems and the consequences of these effects on past, present, and future systems (Oldfield 2005, this volume). Balanced paleoecologic analyses such as Haberle and Chepstow-Lusty (2000), Verschuren et al. (2000), Huntley et al. (2002), Berglund (2003), Haug et al. (2003), Tinner et al. (2003), and Haberle and David (2004) illustrate how Holocene paleoclimatic reconstructions can provide valuable hypotheses about climate-land-use and climate-society relationships. Oldfield (this volume) discusses how such hypotheses can be incorporated into ideas and models about societal structure and processes.

The possible roles of land-cover and land-use change on Holocene climate-change are central to Ruddiman's (2003, 2005b) Anthropocene hypothesis (see above). This challenging hypothesis raises the problems of accounting for changes in Holocene carbon budgets. There are currently major differences between estimates of carbon budgets based on paleoecologic evidence and those based on models (e.g. Pedersen et al. 2003; Joos et al. 2004). These unresolved questions highlight the importance of considering land-cover-climate interactions and of comparing model-based simulations and estimates with empirical evidence for past land-cover change. As discussed above and by Oldfield (2005, this volume), empirical approaches for estimating the extent of deforestation and carbon sequestration and release as a result of human activity need greater rigor and quantification and improved robustness.

The possible impact of climate changes on human societies in the past and the effects of past land-use and land-cover changes on climate directly via changes in albedo, moisture retention, and dust fluxes and on atmospheric gas concentrations and thus on climate are rapidly developing paradigms in Holocene climate research. As Oldfield (this volume) emphasizes, these are topics where much new, critical, and innovative research is urgently needed.

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